Method of softening low-carbon medium-alloy steel



UNITED STATES PATENT OFFICE METHOD OF SOFTENING LOW-CARBON MEDIUM-ALLOYSTEEL John M. Kiefer, Jr., Bedford, Ohio, assignor to United StatesSteel Corporation, a corporation of New Jersey No Drawing. ApplicationAugust 6, 1951; Serial No. 240,613

' 1 Claim. o1.1'4s- 21.5)

1 This invention is concerned with softening lowwhere the steelsstructure includes both ferrite carbon medium-alloy steel. Such steelhas acarand austenite which are both relatively stable bon content ofless than 20% and may contain for relatively long periods of time, theamount of from 3 to 10% of one or more alloys of the class v ferritebeing greatest and the amount of austenite consisting of chromium,nickel, molybdenum, and being the smallest at a temperature just barelymanganese, which elements are soluble in iron above the A61 and AC1temperatures depending on in such amounts and which have the common themethod of heating, and the austenite increasproperty of slowing down thedecomposition of ing in proportion to the ferrite with increasing austnit temperatures until the steel acquires its maxi- The preferred mannerof softening similar mum possible amount of austenite and has littlesteels is to heat them to temperatures in the or no ferrite. Inpracticing the preesnt invenvicinity of 1850 F. followed by slowcooling. tion, the particular temperature to which the This is a ratherexpensive treatment and requires steel is heated theoretically dependsupon the considerable equipment due to the long time specific amount ofcarbides it contains, the best required for cooling at a sufiicientlyslow rate to t mp u being a producing J Sufficient obtain the desiredeffect. I have discovered that austenite to with time dissolve all ofthe carbides, steels of this type can be rendered as soft or softerwhile the remainder of the steels structure reby austenitizing at a lowtemperature as hereinmains ferritic and untransformed by the heating.after described and quenching as they can be by 00 While i is necessaryto allow me t me for the the foregoing treatment. austenite to dissolveall of the carbides, in com- In softening such steel by the method of thmercial work this time is usually provided to an present invention, thesteel is heated to a temadequate degree y the t me qu ed for the Steelperature slightly above its A61 temperature, i. e., to s ak at the mp rau just a ove the Aer not more than about 50 F. above the Aei tempoint toassure its at a n g this temperature perature and soaking at suchtemperature for throughout p all the b de bei g disa sumoient time toequalize th temperature solved in the austenite, the steels structure isand dissolve substantially all the carbides. By predominantly a w -P eStructure consisting such heating, just about sufficient austenitelargely of ferrite with the balance austenite, the is produced todissolve therein about all the carbides being large y d so d in thelatter, as carbides that are in the steel, while the re i 30 described.Furthermore, it is ordinarily sufiicient der of the structure remainsuntransformed to at Steel 0f the yp W Which s i ve and relatively stableas ferrite which is relatively tion is concerned to just as little aboveits Aei unhardenable by quenching. A81 temperature temperature as can bedone while assuring that of the steel is determinable, in so far as thisthe steel has actually been heated above its Ael method is concerned, byaustenitizing a sample temperature so that it has acquired at leastsome. of the steel under question, quenching it violently austenite. Insuch a steel this usually provides to render it martensitic, whereby ithas a highly a structure consisting of from 10 to 40% austenite stressedmetastable structure, and then reheating with the balance untransformedferrite. The the steel to various temperatures, in the vicinity ratio ofthese phases does not change when comwhere the Aei temperature isanticipated, and mercially practicable soaking times are considquenchinguntil the first appearance of untemered, it being possible to hold thesteel at tempered martensite in a background of martensite perature forseveral hours without any great tempered by reheating. The temperaturewhere change occurring in the ratio between the relauntemperedmartensite is first discernible may tively Smaller amou t of au n t nd hre t be taken as the Ael temperature of the steel. amount of ferriteprevailing when thetempera- Due to the accuracy of control required andnec-" ture was first attained. essary, it is preferred to the A01 pointfor obtain- The hot steel may now be cooled to permit its ing thelimited transformation desired although handling, and any rate ofcooling may be used as an alternative method the steel may be heatedwithout making the steel hard and brittle. In to just above its AC1point and immediately withthe case of many shapes ordinary air coolingis drawn from the furnace to achieve the desired satisfactory. However,quick cooling of some sort transformation. is generally desirable in theinterest of saving Between such temperatures and the higher time and ifdesired, the steel may be quenched. temperatures where the steel becomesas wholly pm An actual quenohingis of advantage since it keepseustenitic as t can be made, there is a range the elements fromdiffusing into the ferrite and it transforms the relatively small amountof austenite included in the structure of the steel so that this portionbecomes a hard constituent such as fine pearlite or martensite which ishence more easily spheroidized by tempering. This does not harden thesteel but in fact softens it, because the amount of ferrite is so largethat it overbalances the amount of austenite resulting from the desiredheating. This is the reason why it is desirable in the case of thepresent invention to select a temperature that is only very slightlyabove the A61 temperature, since this provides for maximum retention ofthe ferrite and the production of just sufficient austenite to dissolvemost of the carbides. The type of steel to which this invention isapplicable contains a sufficiently small amount of carbides to make itpracticable to follow what has been discussed, but in the case ofmaterially higher carbon contents it becomes necessary to produce somuch austen-- its, by going to a suitable higher temperature, as toleave so little untransformed ferrite that the steel is actuallyhardened.

The improved results obtained by my improved method of softeninglow-carbon medium-alloy steel are aptly shown by the following testsconducted with specimens of hot-rolled and normalized SAE 3312 steelcontaining:

Percent Carbon 0.15 Manganese .38 Silicon .23 Nickel -i 3.95 Chromium1.40

This steel had an Aei temperature of 124.0 F. and in the normalizedcondition a Brinell hardness of 382. Normalized specimens of the steelwere heated to the temperatures tested in the following Table A, held atsuch temperatures for minutes and air cooled with the following results:

Table A Brinell Temperature: hardness 1,100 E 258 1.250" F 238 1,275 F;255 1,300 F; 265

Similar results were obtained on AISI 2315 steel containing:

Percent Carbon -i 0.14 Manganese .45 Silicon .25 Nickel 3.75

This steel had a Brinell hardness in the hotrolled and normalizedcondition of 159 and an A61 temperature of 1215 F. Specimens thereofwere heated to the following temperatures and held at the temperaturesindicated for 2 hours and air cooled with the results shown in thefollowing Table B:

i Table B Brinell Temperature: hardness 1,l F 143 1,220 F 1,2'I5 F 131In both the foregoing cases the specimens were normalized prior totreatment to insure uniformity of results. .However, such priortreatment is not essential to attainment of the objects of thisinvention.

The foregoing data shows that instead of the expected hardening due tothe formation of hard transformation products such as martensite or finepearlite, the steel is actually softened if treated in accordance withthis invention. As a result of such treatment, the steel has a structureconsisting mainly of ferrite, so that the steel has the ductility andmalleability imparted by this structure, which overbalances the minoramount Of hardened structure. In commercial work, this hardenedstructure will usually be a hardened metastable form of stressed ferriteand. carbides, such as martensite or fine pearlite. This structure canbe spheroidized relatively easily by conventional spheroidizingtreatment if further softening is desired.

This is a continuation-in-part of my copending application, Serial No.107,377, filed July 28, 1949, now abandoned which was a continuation ofmy prior application, Serial No. 523,365 filed February 21, 1944, nowabandoned.

I claim:

A method of softening low-carbon mediumalioy steel containing less than20% carbon and from 3 to 10% of at least one of the alloying elements,chromium, nickel, molybdenum and manganese, which includes heating thesteel to a temperature just above its A81 temperature to produce justenough austenite to dissolve therein about all its carbides while theremainder of its structure remains untransformed and relatively stableas ferrite; holding at such temperature for a suflicient time todissolve substantially all of the carbides in said austenite, and thenquenching the steel at a rate wherein the austenite with the dissolvedcarbides will transform to a hard constituent composed of a hardenedmetastable form of stressed ferrite and carbides, whereupon the coolsteel is mainly unhardened due to its being composed principally offerrite which overbalances the minor amount of the hard constituent.

JOHN M. KIEFER, JR.

References Cited in the file of this patent UNITED STATES PATENTS Ironand Steel Engineer, November 1947, pages 55 and 57.

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